A set is an unordered collection which does not allow duplicate elements.

a = [1,2,3,3,4,'yes','no']
print a
# [1, 2, 3, 3, 4, 'yes', 'no']
print set(a)
# set([1, 2, 3, 4, 'yes', 'no'])

The number 3 appeared only once in the set. Searching for items in set is also easy:

a = [1,2,3,3,4,'yes','no']
print set(a)
print 'yes' in a	# true
print 'n' in a		# false
print 2 in a		# true

Set Arithmetic

a = set('python')
b = set('php')
print b			# unique letters
# set(['p', 'h'])
print a - b		# difference
# set(['y', 't', 'o', 'n'])
print a | b		# union
# set(['p', 't', 'y', 'h', 'o', 'n'])
print a & b		# intersection
# set(['p', 'h'])
print a ^ b		# symetric difference
# set(['y', 't', 'o', 'n'])